1. Field of the Invention
The present invention relates to a packaging substrate. More specifically, it relates to a method for fabricating metal bumps by electroplating on a flip-chip substrate for flip chip packaging.
2. Description of the Related Art
Ball Grid Array (hereinafter referred to as BGA) packaging is widely applied to package the integrated circuits of chip sets or graphic chips, etc. Conventionally, the BGA packaging has tin balls provided on the bottom surface of a substrate and arranged in a form of an array. The balls serve as the leads or pins (conductive media) between a chip (or IC) and a circuit board, replacing the conventional lead frames. The BGA packaging can provide not only more pins but also more space between every two adjacent pins than that of conventional packaging, under the same size of substrate. In addition, BGA packaging provides superior heat dissipation and electrical conductivity by providing shorter conducting paths between the chip and the circuit board.
According to the raw material of the substrate, BGA substrates are divided into three categories: Plastic BGA (PBGA), Metal BGA (MBGA), and Tape BGA (TBGA). The PBGA substrate is made of organic materials such as compounds of BT resin and glass fiber. It is the most popular BGA substrate in the packaging industry.
To meet the need for shrinking package sizes and growing lead counts, flip chip and ball grid array (BGA) technologies have become increasingly popular. Flip chip relates to the attachment of an integrated circuit to a substrate while BGA relates to the attachment of a substrate to a printed circuit board or the like. Flip chip BGA packages (FCBGA), which combine the two technologies, are relatively small and have relatively high lead counts.
One conventional method for creating FCBGA packages is shown in FIG. 1. Solder pads are formed on an integrated circuit 102 and the integrated circuit 102 is bonded to solder bumps on one side of a substrate 106 to form conductive solder balls 104, and underfill 103 is then dispensed between gaps between the integrated circuit 102 and the substrate 106 and then cured, and solder balls 108 are attached to the other side of the substrate 106 to connect to another printed circuit board.
The substrate for flip chip packaging is usually formed by fabricating bumps on an insulating substrate with wiring thereon. Conventionally, the bump pads are fabricated by solder paste printing to form conductive bumps on the substrate, which is generally called pre-soldering. One disadvantage of the solder printing process is the complex steps. Another advantage is the solder paste printing process can only be performed on tools specified for printed circuit boards. Also, the reliability of bump bonding depends on the material of bump pads, conventionally Snxe2x80x94Pb alloys or the like formed by solder paste printing.
To simplify the above process for fabricating bump pads on a substrate for flip chip packaging, an object of the present invention is to provide a process for fabricating metal bumps on a flip-chip substrate by electroplating.
Another object of the present invention is to provide a process for fabricating a flip-chip substrate with copper bumps thereon. The copper bumps will form stable Snxe2x80x94Cu intermetallic compounds (IMC) with bonding pads on IC chips after packaging and the fabricating process of bumps on a flip-chip substrate is also simplified.
To achieve the above objects, the present invention provides a process for fabricating conductive bumps, comprising the following steps. A substrate is provided with conductive points thereon and a conductive film is formed over the surface of the substrate to cover the conductive points. A photoresist layer is formed over the conductive layer and patterned to form openings exposing the underlying conductive points. An electroplating is performed to fill the openings as metal bumps. Finally, the photoresist layer and the conductive film are removed and a solder mask layer is formed over the substrate and exposing the metal bumps.
The present invention further provides a process for fabricating a flip-chip substrate with copper bumps thereon, comprising the following steps. A flip-chip substrate is provided with conductive points thereon and a conductive film is formed over the surface of the flip-chip substrate to cover the conductive points. A photoresist layer is formed over the conductive layer and then patterned to form openings exposing the underlying conductive points. A copper plating is performed to fill the openings as copper bumps. The photoresist layer and the conductive film are removed. Finally, a solder mask layer is formed over the flip-chip substrate, exposing the copper bumps, and an anti-oxidation treatment is performed to finish the exposed copper bumps.
According to the present invention, the electroplating process can deposit conductive metal bumps in predetermined openings, which provides a simpler and easier process to take the place of conventional solder paste printing process. Also, a more stable bonding will be achieved between copper bumps on a flip-chip substrate formed according to the invention and bonding pads on a chip by forming the Snxe2x80x94Cu intermetallic compounds (IMC) after packaging.